The present invention relates to telemetry systems for remote patient monitoring. More specifically, the invention relates to the use of leads, such as ECG (electrocardiograph) leads, to provide antennas for ambulatory or other remote telemetry devices.
A variety of patient monitoring systems exist that allow the physiologic data of patients within a medical facility to be monitored remotely using wireless communications. These systems commonly include remote transmitters or transceivers that collect, and transmit over a wireless channel, the physiologic data of respective patients. This physiologic data may include, for example, real-time electrocardiograph (ECG) waveforms, SpO2 levels, and non-invasive blood pressure readings. The transmitted physiologic data is conveyed to one or more centralized monitoring stations within the medical facility. From such a monitoring station, a clinician can visually monitor the physiologic status, in real time, of many different patients. The monitoring stations may also run automated monitoring software for detecting and alerting personnel of certain types of physiologic events, such as the occurrence of a cardiac arrhythmia condition.
To enable patients to be monitored while ambulatory, some systems include battery-powered remote transceiver devices that are adapted to be worn by or attached to patients while ambulatory (xe2x80x9cambulatory transceiversxe2x80x9d). Each ambulatory transceiver attaches to a patient by a pouch or other attachment device, and senses the patient""s physiologic data via a set of ECG leads (and/or other types of sensor leads). In one common design, each lead wire of the ECG lead set is constructed of a shielded wire (typically coaxial) comprising an inner conductor surrounded by a mesh shield. The inner conductor electrically connects an ECG sensor to the ambulatory transceiver""s sensor circuitry, and is used to carry ECG signals. The outer shield protects the ECG signals from radio frequency (RF) interference. In other designs, each lead wire is an unshielded, single-conductor wire.
In some prior art designs, selected portions of the ECG lead wires are used as the RF telemetry antenna. For example, in one design in which the lead wires have outer shields that are a fractional length of the total wire length, the shields of multiple lead wires are connected together to form the antenna. In another design, the multi-strand conductor of the RL (right leg) lead wire is used as the antenna. An important benefit of these designs is that they eliminate the need for a dedicated antenna mounted to or inside the transceiver""s housing. In addition, a lead antenna can provide a somewhat larger aperture, and thus better RF link performance, than a housing mounted antenna.
One problem with existing designs is that the coaxial ECG lead wires tend to be relatively stiff in comparison to other types of wires. As a result, the leads cause discomfort to patients and tend to lack durability.
Another problem with existing designs, and particularly with ambulatory transmitter and transceiver designs, is that data transmissions are highly susceptible to attenuation caused by the patient""s body or nearby objects. This problem is frequently experienced when the patient is in bed. For example, if the patient rolls over on top the antenna (dedicated or ECG lead), the patient""s body may block signal transmissions to and from the ambulatory device. Further, the patient""s position in bed may cause a portion of the antenna to be positioned close to a bed rail or other grounded metal object, causing the entire antenna to de-tune. In these situations, the patient""s real time physiologic data typically can not be remotely monitored with sufficient reliability.
Yet another problem with existing telemetry devices, and other types of devices that receive signals from an ECG lead set, is that they do not provide an adequate solution to the problem of protecting against defibrillation pulses. For example, some designs merely use current-limiting resistors connected along the ECG signal lines. These resistors tend to be large, high-power components, and tend to increase the manufacturing cost of the device while providing only limited protection.
The present invention addresses these and other problems with prior art designs by providing several inventive features that may be used individually or in appropriate combination. One such feature involves the replacement of some or all of the conventional lead wires with lead wires having two side-by-side conductors. In each such lead, one of the two conductors is used to carry ECG signals, and the other is used as an antenna element. An important benefit of this design feature is that the leads are generally more flexible, and lighter in weight, than coaxial leads. As a result, the leads provide greater comfort to patients. Further, in comparison to typical lead wire antenna designs in which the coaxial shield extends only a few inches, the use of an antenna conductor that extends substantially the entire length of the lead wire (as in the preferred embodiment) provides improved antenna performance. Additional benefits include greater lead durability and lower cost of lead material. This and the other features of the invention may also be used with other types of lead sets for sensing physiologic data, such as EEG lead sets and leads sets with SpO2 and oscillometric blood pressure sensors.
Another feature involves statically or dynamically dividing the set of ECG or other leads into two or more groups to provide two or more corresponding telemetry antennas. For example, in a lead set with five ECG leadwiress, the antenna portions of the RL (Right Leg) and C (Chest) leads may be electrically connected to form a first antenna, and the antenna portions of the LA (Left Arm), LL (Left Leg) and RA (Right Arm) leads may be interconnected to form a second antenna. The leads may, for example, be constructed with conventional coaxial lead wires in which the outer shields are used as the antenna portions, or may be constructed with wires having side-by-side conductors as described above. To provide diversity, a control circuit within the transceiver selects between the multiple antennas, preferably based on observed characteristics of received RF transmissions. Thus, for example, when one antenna produces data errors as the result of a lead touching a bed rail, the control circuit may switch to an antenna that does not use the affected lead. In embodiments in which the telemetry device transmits but does not receive data via the antenna used to transmit, the antenna diversity may be selected using antenna impedance measurements (as described below). Alternatively, the telemetry device may simply transmit the same data separately using each antenna to provide redundant transmissions.
In one embodiment, the ECG leads are statically grouped to form the multiple antennasxe2x80x94preferably by fixed electrical connection of antenna conductors within the lead set""s connector plug. In another embodiment, the antenna portions of the ECG leads are connected to an electrical switch, such as a matrix switch capable of selecting any combination of one or more ECG leads to use as the antenna. The switch is controlled by a control circuit that dynamically selects the one or more leads to use to form the antenna based on observed signal characteristics and predefined selection criteria.
Another feature, which may be used alone or in combination with the above-mentioned features, involves the use of an impedance detector to monitor the impedance of an ECG-lead antenna (or an antenna that includes conductors within other types of leads). The output of the impedance detector may be used to control an impedance matching circuit to maintain the ECG lead antenna in a tuned state. For example, when the antenna""s impedance changes as the result of proximity to a bed rail, the antenna""s impedance matching circuit may be dynamically adjusted to maintain the antenna in an optimal state.
The antenna impedance measurements may additionally or alternatively be incorporated into the decision logic used to select an antenna. For example, in one embodiment, an impedance detector is integrated with the above-mentioned matrix switch, and is used to separately monitor the impedance of the antenna portion of each ECG or other lead. These impedance measurements are used (preferably in combination with received signal-quality measurements) to select the lead or leads to use to form the antenna. For example, when the impedance associated with a particular lead falls outside of a predefined range, that lead may automatically be excluded from potential use as or within an antenna.
Another feature of the invention involves using the multiple antenna conductors of the coaxial or non-coaxial leads as elements of a phased antenna array. In one embodiment of a transceiver system, each such antenna conductor is coupled to a respective phase shifter capable of adjusting the phases of signals received and radiated by that antenna conductor. During receive events, a phase detection circuit monitors the phases of the respective RF signals received by the antenna conductors, and controls the phase shifters to compensate for phase differences. During transmission events, the phase shifters are used to effectively steer the beam formed by the antenna array in the direction of a receiving station and/or to reject an interference source. The antenna beam may also be steered passively (without transmitting) to locate a base station.
Another inventive feature, which similarly may be used alone or in combination with the aforementioned features, is an improved circuit for protecting the remote transceiver""s circuitry, or the circuitry of another type of device that receives signals from an ECG lead set, from damage caused by defibrillation pulses. In a preferred embodiment, the circuit includes a low capacitance, transient voltage suppression (TVS) circuit connected between the ECG signal path and ground, and further includes a current-limiting resistor connected in-line along the signal path. Separate protection circuits of this type may be provided along each ECG signal path. The use of low capacitance TVS circuits allows small, low cost, surface-mount current limiting resistors to be used in place of the relatively large current-limiting resistors used in conventional designs.